    The background art of the present invention is described in the following references.    [Patent Reference 1] Japanese Examined Patent Publication (Kokoku) No. 45-28728    [Patent Reference 2] Japanese Examined Patent Publication (Kokoku) No. 46-847    [Patent Reference 3] Japanese Unexamined Patent Publication (Kokai) No. 58-46118    [Patent Reference 4] Japanese Unexamined Patent Publication (Kokai) No. 58-46119    [Patent Reference 5] Japanese Unexamined Patent Publication (Kokai) No. 61-19816    [Patent Reference 6] Japanese Unexamined Patent Publication (Kokai) No. 2003-82543    [Patent Reference 7] Japanese Unexamined Patent Publication (Kokai) No. 2003-41444    [Patent Reference 8] Japanese Unexamined Patent Publication (Kokai) No. 2003-41462    [Patent Reference 9] Japanese Unexamined Patent Publication (Kokai) No. 3-213518    [Patent Reference 10] Japanese Unexamined Patent Publication (Kokai) No. 49-72485    [Patent Reference 11] Japanese Unexamined Patent Publication (Kokai) No. 50-116708    [Patent Reference 12] Japanese Unexamined Patent Publication (Kokai) No. 9-316744
It has heretofore been well known that natural fibers such as cotton, wool and feather fibers reversibly change their forms and crimp ratios as humidity changes. Investigations have long been made to make synthetic fibers have such functions. For example, Patent References 1 and 2 have already proposed side-by-side conjugate fibers prepared from a nylon 6 and a modified poly(ethylene terephthalate). Because known conjugate fibers show very small changes in reversible crimp ratios when moisture changes, they have not been put into practical use.
Patent References 3 and 4, and the like, have proposed conjugate fibers prepared under improved heat treatment conditions. Moreover, Patent References 5 to 8, and the like, have proposed conjugate fibers prepared by applying the above conventional technologies. However, the actual situation is that the conjugate fibers obtained by applying the above conventional technologies decrease their crimp ratio changes when subjected to steps, such as dyeing and finishing. As a result, conjugate fibers have not been put into practical use.
In contrast to the above technologies, Patent Reference 9 discloses an attempt to improve the above problems wherein a polyester component and a polyamide component are conjugated in a flat-like state, and a polyamide having a high moisture absorption ratio as a nylon 4 is used as the polyamide component. However, the productivity stability of the nylon 4 is poor, and the crimpability is impaired by heat treatment. Therefore, there is also a restriction on the practical use of such a conjugate fiber.
On the other hand, in addition to the recent problem of ensuring stabilized quality in the yarn productivity and finish texturing, the “see-through” of a fabric prepared from a conjugate fiber has recently become a problem to be solved, among the diversified properties the conjugate fiber is required to have. That is, when a conventional woven or knitted fabric formed from a synthetic fiber or a natural fiber is used for swimwear, sportswear, or the like, the fabric is likely to become “see-through”, when wetted with water, and windbreaking and warmth-retaining properties also become poor. Moreover, there is also a demand for a filament yarn and a fabric that has bulkiness and a silk-like touch.
On the other hand, fibers having bulkiness such as a spun yarn have been examined. For example, Patent Reference 10 discloses a method of obtaining a frosty tone fiber by interlacing two types of yarns that have been prepared by spin combining, and heat treating the interlaced yarn. Moreover, Patent Reference 11 discloses a method of spin combining two types of polymers differing from each other in dye-affinity. Furthermore, Patent Reference 12 discloses a method of obtaining a fiber having a moiré tone appearance by combining two types of yarns differing from each other in orientation, in a drawing step so that the dye-affinity difference is utilized. A spun-like woven or knitted fabric having a moiré tone or a frosty tone can be obtained from combined yarns prepared by the above-proposed methods. However, a woven or knitted fabric having a wool-like bulge cannot be obtained. Of course, the above combined yarn has no properties of changing crimps in accordance with the amount of humidity, like wool.